20 TRANSACTIONS OF THE [OCT. 19^ 



very minute. It builds idiomorphic crystals bounded by the do- 

 decahedron, is usually traversed by innumerable cracks, and often 

 shows a very perfect cleavage parallel to the the crystal boundary. 

 In size the individuals vary from the smallest microscopic specks 

 to an inch or more in diameter. Inclusion of quartz, pj'roxene 

 and hornblende are common, the former sometimes assuming a 

 regular order parallel to the planes of the dodecahedron. Be- 

 sides these, the garnet is often sprinkled with exceedingly fine 

 acicular crystals of yellowish color and a high refraction which 

 greatly exceeds that of the inclosing minerals. They are termi- 

 nated by a low pyramid and show bright polarization colors. 

 These, characteristics together with straight extinction would 

 indicate that the mineral is rutile though Riess thought it to be 

 zircon. Determinations of the index of refraction on the garnet 

 from Silberbach (]),and Fallser Hiihe (2), gave the following 

 values : 



No. 1. No. 2. 



Li. lisht, 1.7568 1.7647 



Na. light, 1.7610 1.7700 



Tl. light, 1.7650 1.7747 



Spec, gray., 3.758 



The figures under No. 1 agree with the chemical composition 

 as determined by Y. Gerichten, who found: SiO., 43.16, AUOg 

 23.07, FeO 14.60*, MnO 0.91, CaO 13.84, MgO 6.05, total, 101.63 

 — an isomorphic mixture of almandine, grossularite and pyrope. 

 The relatively high percentage of SiO., is due to quartz inclusions. 

 In No. 2 the almandine molecule probably predominates, giving 

 higher refraction and greater difterences than No. 1. 



The pyi'oxene possesses a green color ranging from light to 

 dark shades, and has no distinguishable pleochroism. It has been 

 called omphacite, and has been considered a special type of the 

 augite group. The individuals are usually without idiomorphic 

 boundaries, occurring as small irregular grains, which sometimes 

 are extended along the c-axis but never attain a length of more 

 than a few millimeters. Extinction angles in the prism zone range 

 from 0° to 40°. It is seldom that any clearly defined cleavage 

 lines can be seen in a slide, but b}' pulverizing a specimen of the 

 rock from Silberbach, pieces of omphacite were found, which 

 gave on the goniometer 87° and 93° for the prism-angle. In- 

 clusions are very common, and consist of garnet, hornblende, 

 cyanite, quartz and rutile. 



According to the analj^ses that have been made, the pyroxene 

 is unusually high in alumina, that found near Eppenreuth yield- 

 ing 9.69 per cent., and the others above 8.5 per cent. But it is 

 doubtful whether these values are to be relied upon as being 



